using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

using System.Collections.Specialized;

using System.ComponentModel;

using System.Linq;

using System.Runtime.CompilerServices;

​

public class Program

    {

        public static void Main()

        {

            var inventoryManagement = new Program();

​

            //Customer purchases 49 Ball point pens

            //after the purchase only 1 will be left in inventory. So, RefillRequest will be triggered

            inventoryManagement.CheckOut(1, 49);

        }

​

        public ObservableCollection<Item> Inventory { get; set; }

        public Order Orders { get; set; }

​

        public Program()

        {

            Orders = new Order();

            Inventory = new ObservableCollection<Item>();

            Inventory.CollectionChanged += InventoryChanged;

            InitialiseInventory();

        }

​

        public void InitialiseInventory()

        {

            Inventory.Add(new Item() { ItemId = 1, Name = "Ball Point Pen", Price = 5, StockInHand = 50 });

            Inventory.Add(new Item() { ItemId = 2, Name = "Notebook", Price = 5, StockInHand = 20 });

            Inventory.Add(new Item() { ItemId = 3, Name = "Pencil", Price = 5, StockInHand = 100 });

            Inventory.Add(new Item() { ItemId = 4, Name = "Eraser", Price = 5, StockInHand = 30 });

        }

​

        private void CheckOut(int itemId, int units)

        {

            var item = this.Inventory.FirstOrDefault(x => x.ItemId == itemId);

            if (item != null && item.StockInHand > units)

                item.StockInHand = item.StockInHand - units;

        }

​

        private void InventoryChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)

        {

            if (e.Action == NotifyCollectionChangedAction.Remove)

            {

                foreach (Item item in e.OldItems)

                {

                    //Removed items

                    item.PropertyChanged -= ItemPropertyChanged;

                }

            }

            else if (e.Action == NotifyCollectionChangedAction.Add)

            {

                foreach (Item item in e.NewItems)

                {

                    //Added items

                    item.PropertyChanged += ItemPropertyChanged;

                }

            }

        }

​

        public void ItemPropertyChanged(object sender, PropertyChangedEventArgs e)

        {

            if (e.PropertyName == "StockInHand")

            {

                var item = sender as Item;

                if (item == null) return;

​

                if (item.StockInHand < 5)

                    Orders.RefillRequest(item.ItemId, 100);

            }

        }

​

        public class Item : INotifyPropertyChanged

        {

            public int ItemId { get; set; }

​

            public string Name { get; set; }

​

            public int Price { get; set; }

​

            private int stockInHand;

            public int StockInHand 

            {

                get { return stockInHand; }

                set 

                {

                    this.stockInHand = value;

                    NotifyPropertyChanged();

                }

            }

​

            public event PropertyChangedEventHandler PropertyChanged;

            private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")

            {

                if (PropertyChanged != null)

                {

                    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

                }

            }

        }

​

        public class Order : List<Order>

        {

            int ItemId { get; set; }

            int Units { get; set; }

​

            public void RefillRequest(int itemId, int orderCount)

            {

                //Check if order already exists

                if (!this.Any(x => x.ItemId == itemId))

                {

                    //Send request to place order

                    this.Add(new Order() { ItemId = itemId, Units = orderCount });

                    Console.WriteLine("Reorder request is triggered for ItemId : " + itemId);

                }

            }

        }

    }